A steering system for an automobile widely adopts a so-called power steering apparatus for having a steering assist performed by use of an external power source.
Generally, the power source for the power steering apparatus has hitherto involved the use of a vane type hydraulic pump that is driven by an engine in many cases. This type of power steering apparatus has, however, a large drive loss of the engine as the hydraulic pump is driven without interruption (the drive loss is on the order of several hp (horsepower) to several tens of hp when at the maximum load). It is therefore difficult to adopt this power steering apparatus for mini-sized motor vehicles, etc. having a small displacement, and a traveling fuel efficiency of even an automobile having a comparatively large displacement inevitably decreases down to an unignorable degree.
Such being the case, over the recent years, an emphasis for solving these problems has been placed on an electric power steering (which will hereinafter be abbreviated to EPS) apparatus employing an electric motor as a power source. The EPS exhibits features, wherein the EPS uses an on-vehicle battery as a power source of the electric motor and therefore has no direct drive loss of the engine, a decline of traveling fuel efficiency (which is a drive loss of the engine related to an alternator) can be restrained as the electric motor is started up only when in the steering assist, and in addition execution of the electronic control can be highly facilitated. Note that the EPS is classified into a column assist type, a rack assist type, etc., depending on where the electric motor is installed, however, at the present the most-widespread type is the column assist type having merits in terms of a manufacturing cost, a space, and so on. It should be noted that a general version of the column assist type electric power steering apparatuses is that the steering shaft, as disclosed in Japanese Patent Application Laid-Open No. 2000-855596, gets collapsed upon receiving a predetermined impact load in order to relieve a secondary collision of an occupant (driver) with the steering wheel when an automotive vehicle encounters the (primary) collision.
On the other hand, the steering apparatus of the automobile is used (steered) by a majority of unspecified drivers, and it is therefore desirable that a position of the steering wheel can be adjusted corresponding to an individual physique, a driving posture, etc. For meeting such a request, there increases the number of trucks or lorries and the like without being limited to the passenger cars, which adopt a tilt adjustment mechanism and a telescopic adjustment mechanism. The tilt adjustment mechanism is a mechanism for adjusting a position of the steering wheel in up-and-down directions. The tilt adjustment mechanism is constructed of a tilt pivot for supporting the steering column in a swingable manner, a tilt lever for fixing the steering column in a desired position (at a desired tilt angle), and so on. Further, the telescopic adjustment mechanism is a mechanism for adjusting the position of the steering wheel in the front-and-rear directions (in the axis-directions of the steering shaft). The telescope adjustment mechanism is constructed of a telescopic portion of a double-pipe type or the like that serves for lengthening or shortening the steering shaft, a telescopic lever for fixing the steering shaft in a desired position (with a desired lengthening or shortening quantity), and so forth.
The column assist type electric power steering apparatus provided with both of the tilt adjustment mechanism and the telescopic adjustment mechanism, is exemplified by what is disclosed in Japanese Patent Application Laid-Open Nos. 11-301492 and 2000-211533, etc. Those electric power steering apparatuses, however, present a problem, wherein smooth operations of the telescopic adjustment and the tilt adjustment were not attained due to such a layout that the tilt pivot is disposed at a lower end of the steering column.
For example, as illustrated in FIG. 5, a comparatively heavy electric assist mechanism 17 structured of an electric motor, a speed reducing gear, etc. is attached to a steering column 1. When making the tilt/telescopic adjustments, however, a weight of this electric assist mechanism 17 is applied to between the tilt pivot 31 and the steering wheel, and consequently a large bending stress acts on a fitting slide portion 71 between an upper column 13 and a lower column equivalent member 15 that configure a telescopic mechanism. As a result, this larger bending stress and incapacity of increasing a length L of the fitting slide portion 71 because of an existence of the electric assist mechanism 17, are combined into an occurrence of torsional blocking between the upper column 13 and the lower column equivalent member 15, with the result that the telescopic adjustment operation can not be smoothly performed. This unsmoothed operation is conspicuous because of a single-hand telescopic operation conducted in a way that releases a lever with one hand while grasping the steering while with the other hand.
In the case of increasing the length L of the fitting slide portion 71 in order to improve this problem, a problem arises, wherein a collapsing stroke of the steering upper shaft 9 decreases as well as a telescopic stroke S, a degree of freedom for the telescopic adjustment decreases, and at the same time safety of the occupant (driver) against the secondary collision is hindered. Moreover, when in the tilting operation, a total weight of the steering column 1 including the electric assist mechanism 17 acts on the tilt pivot 31 as a fulcrum, and hence a smooth tilting operation still remained unperformable even by use of a tilt assist spring 65 having large spring force.